Outboard motors and recreational vehicles such as snowmobiles often have electrical systems including a permanent magnet alternator, a battery, various connected loads, and a solid state combination regulator rectifier to change the alternating current output of the alternator to a controlled DC current for maintaining the required charge on the battery. As these vehicles have become more sophisticated, the required electrical loads have in general increased and there has been a corresponding requirement to increase the current carrying capability of the regulators and increase the amount of current obtainable from a given size or weight alternator. Except in some very low power applications, the regulators are in general full wave rectification from either single phase or three phase permanent magnet alternators. Improvements in the alternator output have come recently from such technology as rare earth magnets. This has resulted in many alternator windings running near the upper limit of a temperature capability of the insulation systems of the magnet wire. Regulators commonly used in this type of system include solid state switches, such as silicon control rectifiers and diodes, used to control the current flow from the alternator to the battery and load. These components and the alternator winding must of course be sized so that when the alternator is producing full output, with the switching devices such as silicon control rectifiers always on when instantaneous circuit polarity is such that they can conduct, that both the alternator and the semi conductor components are within their current capabilities, or stated another way within the temperature capabilities of those components. A problem arises if the design of the regulator allows an imbalance in current between the various phases of a three-phase alternator or the positive and negative half cycles of a single-phase alternator.
It has been observed that a full wave rectified single phase alternator operating at high RPM can maintain as much, and in some cases even slightly more, average current output with one of the polarities disabled. In this extreme example, even though the average current to the battery and load remains approximately the same the average current through the switching device and diode that is still in operation is increased 2 to 1 and the true RMS current or effective heating value of the current through the alternator winding is also drastically increased. The forgoing results from an effective phase shift, resulting in conduction of greater than 180 degrees compared to the open circuit voltage waveform of the alternator. Whether a given combination of alternator and regulator operates with a reasonable balance between the possible conducting paths, i.e. polarity or phases, can be influenced greatly by the connected load and the condition or type of battery used. Typically, the manufacturer of a vehicle cannot control the load and the battery condition. Thus, even if initial testing indicated that components share loads as desired it is often found in vehicles at a later time, particularly where batteries may have been substituted or the original battery type deteriorated, that a severe imbalance in the load occurs between for instance the positive and negative half cycle of a full wave rectified single phase, or between the phases of a three phase alternator.